Bobbing along on the research vessel Western Flyer just outside California’s Monterey Bay, marine biologist Karen Osborn and her colleagues were hauling deep-sea fish to the surface for cataloging and a photo shoot. Osborn got her hands on a fangtooth, a self-explanatory creature with a mouth full of nasty, big, pointy teeth, attached to a stout, teardrop-shaped body. Using her custom-built system of strobe lights and a camera mounted above a tank, she could capture the rare specimen for science.
Yet when she put the fangtooth in front of the camera, it turned into a living black hole—the outline was there, but not the fine details, as if the fish was devouring light. “I was trying to take pictures of it, and I was just getting these silhouettes,” Osborn says. “They were terrible.”
This wasn’t her first photo shoot with a deep-sea fish, so it couldn’t be operator error. But wait a second, Osborn figured. “I had tried to take pictures of deep-sea fish before and got nothing but these really horrible pictures, where you can't see any detail,” she says. “How is it that I can shine two strobe lights at them and all that light just disappears?”
It disappears because the fangtooth, along with 15 other species that Osborn and her colleagues have found so far, camouflage themselves with “ultra-black” skin, the deep-sea version of Vantablack, the famous human-made material that absorbs almost all the light you shine at it. These fish have evolved a different and devilishly clever way of going ultra-black with incredible efficiency: One species the researchers found absorbs 99.956 percent of the light that hits it, making it nearly as black as Vantablack.
“We had no idea there were any fishes at all that were ultra-black,” says Duke University biologist Alexander Davis, lead author on a new paper in Current Biology describing the findings. “As far as we knew, the only vertebrates that were ultra-black were these birds-of-paradise and a couple other bird species. It was the first case we have of something this black really being used as camouflage as well.”
You might be wondering why, if the sun’s photons don’t penetrate past 200 meters deep, these fish would need to camouflage themselves with some of the blackest black in the animal kingdom. The reality is that the ocean’s depths are actually aglow with light, in the form of bioluminescence produced by critters clear across the tree of life, from bacteria to fish to squid. (The researchers collected specimens down to 2,000 meters deep.)
This light show serves a wide array of purposes. Some species, accustomed to becoming lunch, spew a cloud of luminescent goo that confuses their hunters—or even sticks to the hunters' bodies, marking them for their own predators. Other deep-sea fish throw off luminescent beams from their faces—searchlights that help them find food. Perhaps most famously, the anglerfish uses a glowing lure to draw prey into its toothy maw. And in the total darkness, the males and females of some species must light up like billboards to find each other.
“Imagine yourself in a bunker with no light whatsoever, and you have to find your mate. You hope you have a flashlight, right?” asks Scripps Institution of Oceanography’s Dimitri Deheyn, who studies bioluminescence but wasn’t involved in the new study. “Light is exploding there, but it's biological light. And most of the time it is mainly blue light, because it’s what propagates the farthest in these waters.”
There’s a problem, though: All this light attracts attention, both for prey and predators. If the prey can see an anglerfish’s face lit up by its dangling lure, the jig is up. Same thing if your face has headlights that help you spot your food. “Obviously, you don't want your prey to see that light casting all over your face,” says Davis. “So if your head is now ultra-black, none of that light goes back to the prey, and you’re more likely to get within range.”
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So how do these 16 species of ultra-black fish camouflage themselves so well? It’s all about melanin, the same stuff that gives us humans our skin coloration. The skin of these fishes is positively packed with a layer of organelles called melanosomes, themselves loaded with melanin.
Tons of melanin alone, though, couldn’t make these fish rival the darkness of Vantablack. When you shine a light on a fangtooth or its peers, the melanosomes absorb most of the photons right away. “But what isn't absorbed side-scatters into the layer, and it's absorbed by the neighboring pigments that are all packed right up close to it,” says Osborn. “And so what they've done is create this super-efficient, very-little-material system where they can basically build a light trap with just the pigment particles and nothing else.”
“It's kind of like a gumball machine, where all of the gumballs inside are the right size and shape to trap light within the machine,” adds Davis. “Once it comes in, it bounces around between all of those little balls and doesn't come back out.”
Incredibly, there wasn’t one common ancestor of these 16 Vantafish species—or at least the 16 that Osborn and Davis have discovered so far—that evolved this trick and passed it along to its evolutionary descendants. The species evolved it independently, so they have slightly different ways of going about absorbing light; their melanosomes are arranged differently in the skin.
“Some of them have just a big jumble,” says Osborn. “Some of them have three layers, some of them have two layers. Some of them have really thick layers, some of them have thinner layers. So there's these little, little differences in how they've actually accomplished this between the different species.”
The skin of all of these fishes is specifically tuned to absorb blue and green light, which are the colors of the majority of bioluminescence in the deep. But the species are using their ultra-blackness for diverse purposes, either as the prey hiding themselves from hunters, or as the hunters hiding themselves from prey. Interestingly, one of the species, the threadfin dragonfish, is only ultra-black as a juvenile. Why? Because tiny, defenseless babies need to hide from predators. As adults, once the threadfin dragonfish grow to over a foot long, they don’t have so much to hide from—they’re higher up the food chain.
And get this: The researchers found some species had ultra-black gut linings, likely to keep them from glowing like lanterns when they eat bioluminescent prey. “It's either going to be startled and bioluminescent when it's in your belly because it's freaking out, or as you digest it, those chemicals are going to release and start bioluminescing,” says Osborn. “You don't want to be swimming around with a glowing belly, right? That's just asking for trouble.”
You also don't want to be swimming around with an empty belly. A coral reef teems with masses of fish, giving predators plenty of choices on the menu. But in the vastness of the deep, prey is relatively scarce, so predators are under much more pressure to score a meal. "Ultra-black seems like an extreme—why would they spend so much energy and evolutionary time in making that?" asks Luiz Rocha, curator of fishes at the California Academy of Sciences, who wasn't involved in this research. "If they weren't so ultra-black, they would still be relatively invisible. And the reason I think is because there's so little prey that they have to be very efficient, to the point of being able to catch almost 100 percent of whatever they come in contact with. Because if they don't, they're going to starve." Other quirks of their morphology also betray this desperation: Species like the anglerfish have enormous mouths and bellies, all the better to consume whatever they stumble upon in the darkness.
These fish have evolved a fascinating kind of hybrid approach to ultra-blackness. Consider the ultra-black birds-of-paradise. Their feathers are darkly pigmented to begin with, but they also make use of tiny treelike structures that bounce light around, absorbing more photons. By contrast, the many species of Vantafish use a two-in-one approach to absorb light with the same effectiveness: The melanosomes are the pigment and the structure that eats all that light. “It makes this whole thing a lot simpler,” Davis says. “So if you were trying to mimic it for any sort of technological purpose, now you only need to fabricate one component.”
By contrast, human-made ultra-black materials make use of carbon nanotubes, tiny cylinders that trap photons, which are a pain to manufacture. “You end up with a pretty fragile material—an incredibly black material—but a fragile one,” says Davis. “Here we're basically seeing just a random aggregation of nanoparticles producing something that's almost that black. There's no special structural alignment that you need. In theory, if you can make nanoparticles of the right size and shape, then you should be able to just coat anything with it.”
No one’s yet lined up to commercialize the dark magic of the Vantafish. But perhaps the ultra-black materials of the future may just be skin deep.